Protective circuit for x-ray tube and method of operation

ABSTRACT

A protective circuit and method of operation for interrupting a signal which is applied to an X-ray tube when the signal attains a value which exceeds a maximum rating for the tube. The protective circuit includes a signal generating circuit for developing a limit signal which varies in value with respect to time in accordance with a maximum tube rating signal, a programming circuit for developing a program signal having a value representative of a preselected signal to be applied to the X-ray tube, and a comparator circuit for developing an interrupt signal if the program signal exceeds the value of the limit signal.

United States Patent [1 1 Lombardo et al.

[ PROTECTIVE CIRCUIT FOR X-RAY TUBE AND METHOD OF OPERATION [75]Inventors: Daniel F. Lombardo, Cleveland;

Walter E. Splain, Woodbridge, both of Ohio [73] Assignee:

[22] Filed:

52 us. Cl. ..250 409, 250/416 51 Int. Cl H05g 1/26 581 Field of Search250/93, 103, 95

[56] References Cited UNITED STATES PATENTS 3.290596 12/1966 Bougle250/95 July 17, 1973 Primary Examiner-James W. Lawrence AssistantExaminerC. E. Church Att0rneyWatts, Hoffman, Fisher & Heinke [57]ABSTRACT A protective circuit and method of operation for interrupting asignal which is applied to an X-ray tube when the signal attains a valuewhich exceeds a maximum rating for the tube. The protective circuitincludes a signal generating circuit for developing a limit signal whichvaries in value with respect to time in accordance with a maximum tuberating signal, a programming circuit for developing a program signalhaving a value representative of a preselected signal to be applied tothe X-ray tube, and a comparator circuit for developing an interruptsignal if the program signal exceeds the value of the limit signal.

31 Claims, 6 Drawing Figures XRAY 26 TUBE l I OVERLOAD: XRAY TUBEEYPUSURE pP Z8 souRcs 5074/6/47- L-I 8l OLT5 FLG-S/NGLE 77/8EL/M/7' TUBEm TUBE CURVE L/M/T L/M/T ADJUSTMENT P DECODING GENERATOR c/RcU/T HT-SM-Elf/ 6:5 CIRCUIT 8 U7- M- I LE 1 307 AF- my 16 +ZOV L14 T/ME DECODING R2i" l 1 un t 7711/! E ACC'UMULA TOR Patented July 17, 1973 5 Sheets-SheetTUBE L/M/ TDCOD/NG C /RC 11/ T t t t t BASE EX POSUFPE T/ME INVENTOR.WAL TER E. SPLA/A/ BY DAN/EL F LOMBARDO PROTECTIVE CIRCUIT FOR X-RAYTUBE AND METHOD OF OPERATION CROSS REFERENCES TO RELATED PATENTAPPLICATIONS AND PATENTS U.S. Patent application Ser. No. 743,421, toWalter E. Splain, entitled X-Ray Tube Kilovoltage Control System", filedJuly 9, 1968, and assigned to the same assignee as the presentinvention.

U.S. Pat. No. 3,284,631 to Walter E. Splain, entitled Device forDetermining the Current-Time Output of an X Ray Tube, issued on Nov. 8,1966, and assigned to the same assignee as the present invention.

U.S. Pat. No. 3,502,877 to Walter E. Splain, entitled Grid-ControlledX-Ray Tube Control System, issued Mar. 24, i970 and assigned to the sameassignee as the present invention.

U.S. Pat. No. 3,521,067, to Walter E. Splain, entitled X-Ray TubeCurrent Stabilization, issued July 21, 1970 and assigned to the sameassignee as the present invention.

BACKGROUND OF THE INVENTION This invention pertains to the art ofelectrical circuits for limiting the value of a signal applied to anelectronic device, and more particularly, to a protection circuit forinterrupting a signal which is applied to an X-ray tube if the value ofthe signal exceeds a predetermined value.

In the operation of X-ray equipment, a very high voltage potentialsignal, for example 125 kilovolts, is applied to the anode of the X-raytube. When operated at this voltage potential, a current of 250milliamperes may flow through the X-ray tube. The resultant input powerapplied to the tube will then be in excess of 30,000 watts. While thisinput power level may be maintained for a relatively short time durationexposure, i.e., on the order of 0.05 seconds, a longer exposure timewill result in permanent damage to the X-ray tube.

X-ray tube protection circuits for use in most radiographic modes ofoperation have heretofore included circuitry for monitoring the value ofthe voltage potential signal which is applied to the X-ray tube, and ifthis signal exceeds a predetermined constant level, the signal isremoved from the tube. These protective circuits have been satisfactoryto a large extent; however, these circuits do not take into account thefact that the maximum input power which may be applied to an X-ray tubedecreases as the elapsed exposure time increases.

Also, in the operation of X-ray equipment, the high potential signalapplied to the anode of the tube is adjusted to vary the intensity ofX-rays which are produced by the tube. With different X-ray procedures,i.e., high speed, low speed, large focal spot, small focal spot,overtable operation and undertable operation, there are differentrequirements as to X-ray intensity and exposure time.

In certain X-ray procedures it is desirable to apply a very highintensity level of X-rays for an extremely short period of time. Withthe above-described tube protection circuits, it was not possible toapply a signal to the tube having a value great enough to produce thedesired X-ray intensity level because the protection circuit wouldremove the signal from the tube since the signal exceeded thepredetermined level.

Also, if the exposure time of the tube is increased beyond a givenperiod of time, the tube will be permanently damaged, even when operatedat a conservative potential level, in view of the fact that the maximuminput power which may be applied to an X-ray tube decreases rapidly withrespect to time. This type of tube damage will occur in the phototimedmode of operation if an X-ray technician merely inadvertently leaves alead apron on the X-ray table at a position in the path of the X-rays soas to prevent the phototiming circuit from deenergizing the X-ray tube.

It has been found to be highly desirable to compare the value of thesignal applied to the X-ray tube not merely with a constant tube limitsignal, but to instead compare the value of the signal applied to theX-ray tube to a limit signal which varies in value in accordance with alimiting parameter of the X-ray tube, such as the maximum input powerwhich may be applied to the tube.

SUMMARY OF THE INVENTION The present invention is directed toward aprotective circuit and method of operation for interrupting theoperation of an X-ray tube whenever a signal applied to the tube exceedsa maximum time-varying rating for the tube, thereby overcoming the noteddisadvantages, and others, of such previous systems.

In accordance with one aspect of the present invention, there isprovided in an X-ray apparatus a protective circuit for limiting thevalue of a signal applied to an X-ray tube. The protective circuitincludes a signal generating circuit for generating a limit signal whichvaries in value with respect to time in accordance with the value of amaximum tube rating signal, a programming circuit for developing aprogram signal having a value representative of the value of apreselected signal to be applied to the X-ray tube, and a comparatorcircuit for monitoring the limit and program signals for developing anoutput signal whenever the value of the limit signal attains apredetermined value with respect to the value of the program signal.

In accordance with another aspect of the present invention, the signalgenerating circuit includes a circuit for generating a limit signalwhich decreases in value with respect to time in accordance with thevalue of a decreasing maximum power rating with respect to exposure timeof the X-ray tube.

In accordance with another aspect of the present invention, the signalgenerating .circuit includes a variable control for varying the rate atwhich the limit signal decreases with respect to time.

In accordance with another aspect of the present invention, the signalgenerating circuit includes a circuit for generating a limit signalwhich decreases in ampli tude by a predetermined amount at each intervalfor N intervals of time where N is equal to or greater than five.

In accordance with still another aspect of the present invention, thesignal generating circuit includes a circuit for generating a limitsignal having a plurality of timed portions each being of a differentamplitude.

In accordance with another aspect of the present invention, the signalgenerating circuit includes an actuatable switching circuit forselectively developing a pattern of control signals representative of adesired one of a plurality of predetermined tube rating signals, and asecond actuatable circuit coupled to the actuatable switching circuitfor, upon receipt of a pattern of control signals, actuating the signalgenerating circuit corresponding to the received pattern of controlsignals to thereby generate a limit signal which varies in accordancewith a selected one of the plurality of predetermined tube ratingsignals.

In accordance with still another aspect of the present invention, theprogramming circuit includes a variable circuit for altering the valueof the programmed signal in accordance with the value of a voltage to beapplied to the X-ray tube.

In accordance with still another aspect of the present invention, theprogramming circuit includes a variable circuit for altering the valueof the program signal in accordance with the value of a current and/or avoltage to be applied to the X-ray tube.

In accordance with another aspect of the present invention, there isprovided a method of protecting an X-ray tube in an X-ray apparatus. Themethod includes the steps of generating a limit signal which decreasesin value with respect to time in accordance with a decrease in the valueof the amplitude with respect to exposure time of a maximum signal whichmay be applied to the Xray tube, developing a program signal having avalue representative of the value of a preselected signal to be appliedto the X-ray tube, and comparing the values of the limit signal and theprogram signal and developing an output signal if the value of theprogram signal exceeds the value of the limit signal.

In accordance with another aspect of the present invention, the methodincludes the step of interrupting the signal applied to the X-ray tubein response to the receipt of an output signal.

It is therefore an object of the present invention to provide aprotective circuit for an X-ray tube for interrupting the operation ofthe X-ray tube when the signal applied to the tube exceeds a maximumrating for the tube.

Another object of the present invention is to provide a protectivecircuit for an X-ray tube which monitors the value of a signal to beapplied to the tube and also monitors a time-varying signalrepresentative of a maximum rating for the tube.

Another object of the present invention is to provide in an X-ray systema control circuit for interrupting the signal which is applied to anX-ray tube whenever the value of that signal exceeds the value of amaximum input power rating for the tube.

A further object of the present invention is to provide a protectivecircuit for interrupting the operation of an X-ray tube whenever theinput power applied to the tube exceeds a predetermined time-varyingmaximum power rating for the tube.

A further object of the present invention is to provide a protectivecircuit for developing an output indication whenever the value of asignal applied to the X-ray tube exceeds the value of a maximum ratingfor the tube.

Another object of the present invention is to provide a method ofoperation of a protective circuit for interrupting a signal applied toan X-ray tube whenever that signal attains a value in excess of amaximum rating for the tube.

These and other objects and advantages of the invention will becomeapparent from the following description of the preferred embodiment ofthe invention as read in conjunction with the accompanying drawings andin which:

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an electrical block diagramillustrating in basic form the X-ray tube protective system of thepresent invention;

FIGS. 2 through 5 are electrical schematic diagrams illustrating in moredetail the circuitry of the protective system shown in FIG. 1; and,

FIG. 6 is a graphical representation of a typical curve representativeof the maximum input power to be applied to an X-ray tube as a functionof exposure time.

. DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 illustrates an X-ray tubeprotective system 10 which is generally comprised of circuitry fordeveloping a tube limit signal, circuitry for developing another signalhaving a value representative of the signal to be applied to the X-raytube, and a comparator circuit 12 for developing an interrupt signalwhenever the X-ray tube representative signal exceeds the value of thetube limit signal.

More particularly, the tube limit signal generating circuitry'includes atube limit decoding circuit 14 for developing a pattern of signalsrepresentative of the desired mode of operation, i.e., large focal spot,small focal spot, single speed, triple speed, overtable operation, andundertable operation. The pattern of signals developed by the tube limitdecoding circuit 14 is applied to a tube limit adjustment circuit 16,which is in turn coupled to a tube limit curve generator 18. Also, atime accumulator circuit 20 having a plurality of output circuits fordeveloping a pattern of binary coded decimal signals representative ofelapsed time is coupled to a time decoding matrix circuit 22. Thedecoding matrix circuit 22 is in turn coupled to the tube limit curvegenerator 18.

Thus, the tube limit curve generator 18 generates a signal which variesin value with respect to time in accordance with the value of a maximumtube power rating. This signal is applied to one of the input terminalsof the comparator circuit 12.

The circuitry for developing a signal having a value representative ofthe signal to be applied to the X-ray tube generally includes a firstvariable resistor arrangement VR1 in which the resistance may be variedin accordance with the selected voltage potential to be applied to theX-ray tube, and a second variable resis tance arrangement VR-Z in whichthe resistance may be varied in accordance with the value of the currentto be applied to the X-ray tube. The resistive arrangements VR1, VR-2are connected in series between a positive 20 volt voltage potentialsource and a negative 20 volt voltage potential source. The junctionpoint between the resistance arrangement VR-l and the resistancearrangement VR2 is connected directly to the other input terminal of thecomparator circuit 12.

The output tenninal of the comparator circuit 12 is connected through anamplifier 24 including a feedback path 26 to the input terminal of aninverter 28. The output terminal of the inverter 28 is connected to oneof the input terminals of a NAND gate 30. The other input terminal ofthe NAND gate 30 is connected to circuitry for developing a signalrepresentative of an exposure, and the output terminal of the NAND gate30 is connected to the X-ray tube supply source. Thus, when a signal isdeveloped by the comparator circuit 12 indicative of a tube overloadcondition, an output signal is developed by the NAND gate 30 tointerrupt the X-ray tube supply source thereby removing the highpotential signal from the X-ray tube.

The output terminal of the NAND gate 30 is also coupled through aninverter 32 to one terminal of a monitoring lamp L-l. The other terminalof lamp L-l is connected directly to a positive 28 volt supply source.Thus, the monitoring lamp L-l is energized whenever an interrupt signalis applied to the X-ray tube supply source.

The output terminal of the tube limit generator 18 is connected througha resistor R1 to the positive 20 volt supply source in order to maintainthis terminal at a fixed operating potential prior to the current drainapplied to this terminal by the tube limit curve generator 18 as thetube limit signal is developed. Also, a backup time drive signal isapplied to the time decoding matrix circuit 22 by a conductor AA whichis coupled to the output terminal of an amplifier 34. The input terminalof the amplifier 34 is connected through a resistor R2 to the positive20 volt supply source.

Reference is now made to FIG. 2 which illustrates in more detail thecomparator circuit 12, the amplifier and feedback circuits 24, 26, theinverter circuits 28, 32, the NAND gate 30, and the amplifier 34, aswell as the resistance arrangements VR-l, VR-2. More particularly, theresistance arrangement VR-l generally comprises a plurality of resistorsR3, R4, R5, R6 each having one terminal connected to a common junctionpoint T1. The other terminals of the resistors R3, R4, R5, R6 arerespectively connected to one of the terminals of a plurality ofsingle-pole, single-throw switches S3, S4, S5, S6. The other terminalsof the switches S3, S4, S5, S6 are connected in common to the positive20 volt supply source.

Similarly, the resistance arrangement VR-2 includes a plurality ofresistors R7, R8. R9, R each having one terminal connected in common toa junction point T2. The other terminals of the resistors R7, R8, R9,R10 are respectively connected to one of the terminals of acorresponding number of single-pole, single-throw switches S7, S8, S9,S10. The other terminals of the switches S7, S8, S9, S10 are connectedin common to the negative volt supply source.

The junction point T1 of the resistance arrangement VR-l is connecteddirectly to the base of a PNP transistor Q1 in the comparator circuit12. The base of the transistor Q1 is also connected directly to thecollector of an NPN transistor Q2, and through a capacitor C1 to anoutput terminal AE.

The collector of transistor Q1 is connected through a resistor R3 to anoutput terminal AD, and is also connected directly to the base of atransistor Q3 in the amplifier and feedback circuits 24, 26. The emitterof transistor Q] is connected to the cathode of a diode D1 having itsanode connected in common with the anode of a diode D2. The cathode ofthe diode D2 is connected directly to the emitter of a PNP transistor 04The collector of transistor Q4 is connected directly to an outputterminal AC, and the base of this transistor is connected through theresistor R1 to the positive 20 volt supply source and is also connecteddirectly to the output terminal AE. The commonly connected anodes of thediodes D1, D2 are connected to the collector of a PNP transistor Q5having its base connected through a resistor R4 to the positive 28 voltsupply source. The base of the transistor 05 is connected to the anodeof a Zener diode Z1 having its cathode connected directly to thepositive 28 volt supply source. Also, the base of transistor Q5 isconnected through a resistor R5 to the output terminal AC.

The emitter of the transistor Q2 is connected directly to the junctionpoint T2 of the resistance arrangement VR-2 and the base of thistransistor is connected directly to the common contact of a single-pole,doublethrow relay 36. The normally-closed contact of the relay 36 isconnected through a resistor R6 to the positive 20 volt supply sourceand the normally-open contact of the relay 36 is connected through aresistor R7 to the positive 20 volt supply source. Also connected to thenormally-open contact of the relay 36 is the cathode of a Zener diode Z2having its anode connected directly to the negative 20 volt supplysource. Similarly, the cathode of a Zener diode Z3 is connected to thenormally-closed contact of relay 36 and its anode is connected directlyto the negative 20 volt supply source. One terminal of the relay coil 38of the relay 36 is connected through a resistor R8 to the positive 20volt supply source and the other terminal is connected to a commonjunction point T3. Also, a diode D3, polarized as shown in FIG. 2, isconnected across the terminals of the coil 38 of relay 36.

The common junction point T3 is connected through a diode D4, polarizedas shown in FIG. 2, to an output terminal AB. The output terminal isalso connected through a single-pole, single-throw switch UTL-l toground. The switch UTL-l, upon being closed, actuates circuitry in thetube limiting decoding circuit 14 for undertable and large focal spotoperation.

The junction point T3 is also connected through a diode D5, polarized asshown in FIG. 2, and a singlepole, single-throw switch UTS-l to ground.The switch UTS1, upon being closed, actuates circuitry in the tube limitdecoding circuit 14 for undertable and small focal spot operation.

The collector of transistor Q3 in the amplifier and feedback circuits24, 26 is connected through a resistor R9 to the base of a PNPtransistor Q6, and the emitter of transistor Q3 is connected directly toground. The base of transistor Q3 is also connected through a resistorR10 to the collector of the transistor Q6.

The emitter of transistor Q6 is connected directly to the positive 20volt supply source, the base of this transistor is connected through aresistor R11 to the positive 20 volt supply source, and the collector ofthis transistor is connected through a resistor R12 to ground. Also, thecollector of transistor O6 is connected through a resistor R13 to thebase of an NPN transistor Q7 having its emitter connected directly toground. The collector of the transistor Q7 is connected through a diodeD6, polarized as shown in FIG. 2, to a junction point T4 in the inverter28.

The junction point T4 in the inverter 28 is connected through a resistorR14 to the positive 20 volt supply source and through a resistor R15 tothe base of an NPN transistor Q8. The collector of the transistor Q8 isconnected through a resistor R16 to the positive 20 volt supply source,the emitter of this transistor is connected directly to ground, and thebase of this transistor is also connected through a resistor R17 to theoutput terminal AD. Finally, the collector of the transistor Q8 providesa common junction point TS which is connected to the collector of an NPNtransistor Q9 in the NAND gate 30 and to the cathode of a diode D7 inthe inverter 32.

The anode of the diode D7 is connected through a resistor R18 to thebase of an NPN transistor Q10 having its emitter connected directly toground and its collector connected to one terminal of the monitor lampL-l. The other terminal of the lamp L-l is connected directly to thepositive 28 volt supply source. The base of transistor Q10 is alsoconnected through a resistor R19 to the output terminal AD, and theanode of the diode D7 is also connected through a resistor R20 to thepositive 20 volt supply source.

The base of the transistor Q9 in the NAND gate 30 is connected through aresistor R22 to the output terminal AD and is also connected through apair of seriesconnected resistors R23, R24 to the positive 20 voltsupply source. The junction point between the seriesconnected resistorsR23, R24 is connected to the anode of a diode D8 having its-cathodeconnected to circuitry within the X-ray apparatus for developing anexposure signal. The base .of transistor O9 is connected directly toground and the collector of this transistor is also connected to theX-ray tube supply source and to the anode of a diode D9. The cathode ofdiode D9 is connected directly to ground. In addition, the outputterminal AD is connected directly to a negative 8 volt supply source.

The cathode of diode D9 is connected to an NPN transistor Q10 in theamplifier circuit 34. The base of this transistor is connected throughthe resistor R2 to the positive 20 volt supply source, the collector ofthis transistor is connected directly to the positive 20 volt supplysource, and the emitter of this transistor is connected to an outputterminal AA. Finally, a resistor R25 is connected between the outputterminal AA of the amplifier circuit 34 and ground.

Reference is now made to FIG. 3 which illustrates in more detail thecircuitry within the time decoding matrix circuit 22 and the circuitconnections between the matrix circuit 22 and the timer accumulatorcircuit 20. More particularly, the timer accumulator circuit 20, uponbeing actuated, generates a pattern of binary coded decimal signalsrepresentative of the elapsed time. In other words, once an exposurecycle is commenced, the timer accumulator circuit 20 begins a countingsequence with the pattern of binary signals applied to the outputterminals being changed at preselected intervals of time. The pattern ofsignals which appears on the output terminals of the timer accumulatorcircuit 20 for each time interval is set forth in Table l below.

The output terminals of the timer accumulator circuit 20 are connectedto the input terminals DA through D of the time decoding matrix circuit22. As illustrated in FIG. 3, the decoding matrix circuit 22 generallycomprises a digital-to-analog matrix for actuating selected ones of thenine NPN transistors Q11 through 019.

More particularly, the input terminal BA of the decoding matrix circuit22 is connected to the cathode of a diode D having its anode connectedto a. junction point T5. Seven diodes D1 1 through D17 have their anodesconnected to the junction point T5 and their cathodes respectivelyconnected to the input terminals BC, BE, 80, BI, BK, BM, B0. Thejunction point T5 is connected to the cathode of a Zener diode Z1] andis also connected through a resistor R26 to the output terminal AA.

Similarly, the input terminal BB of matrix circuit 22 is connected tothe cathode of a diode D18 having its anode connected to a junctionpoint T6. The junction point T6 is connected to the cathodes of sevendiodes D19 through D25 having their anodes respectively connected toinput terminals BC, BE, BG, Bl, BK, BM, B0. The junction point T6 isconnected to the cathode of a Zener diode Z10 and is also connectedthrough a resistor R27 to the terminal AA.

In a like manner, the input terminal BD of matrix circuit 22 isconnected to the cathode of a diode 26 having its anode connected to ajunction point T7. The junction point T7 is in turn connected to theanodes of six diodes D27 through D32 having their cathodes respectivelyconnected to the input terminals BE, BG, BI, BK, BM, BO. Also, thejunction point T7 is connected directly to the cathode of a Zener diodeD9 and through a resistor R28 to the terminal AA.

The input terminal BF is connected to the cathode of a diode D33 havingits anode connected to a junction point T8, which is in turn connectedto the cathode of a Zener diode Z8. The junction point T8 is alsoconnected to the anode of five diodes D34 through D38 having theircathodes respectively connected to the input terminals BG, BI, BK, BM,B0. In addition, the junction point T8 is connected through a resistorR29 to the terminal AA.

Similarly, the input terminal BH is connected to the cathode of a diodeD39 having its anode connected to a junction point T9 which is in turnconnected to the cathode of a Zener diode Z7. The junction point T9 isalso connected to the anode of four diodes D40 through D43 having theircathodes respectively connected to the terminals Bl, BK, BM, B0. Inaddition, the junction point T9 is connected through a resistor R30 tothe common terminal AA.

In a like manner, the input terminal BI is connected to the cathode of adiode D44 having its anode connected to a junction point T10 which is inturn connected to the cathode of a Zener diode Z6. The junction pointT10 is connected to the anode of three diodes D45, D46, D47, and is alsoconnected through a resistor R31 to the common terminal AA. The cathodesof diodes D45, D46, D47 are respectively connected to the inputterminals BK, BM, B0.

The input terminal BL of the matrix circuit 22 is connected to thecathode of a diode D48 having its anode connected to the cathode of aZener diode Z5 and to the anodes of a pair of diodes D49, D50. Thecathodes of diodes D49, D50, are respectively connected to the inputterminals BM, B0. In addition, the anode of diode B48 is connectedthrough a resistor R32 to the common terminal AA.

The input terminals BM, B0 are respectively connected to the cathodes ofa pair of diodes D51, D52 having their anodes connected in common to thecathode of a Zener diode Z4. The anodes of the diodes D51, D52 areconnected through a resistor R33 to the common terminal AA.

Finally, the common terminal AA is connected through a resistor R34 tothe anodes of eight diodes D53 through D60 having their cathodesrespectively connected to the input terminals BQ, BN, BL, 3], BH, BF,bd, BB. The anodes of these diodes are also connected to the cathode ofa Zener diode Z12.

The anodes of the Zener diodes Z4, Z5, Z6, Z7, Z8, Z9, Z10, Z11, Z12 arerespectively connected to the base terminals of transistors Q19, Q18,Q17, Q16, Q15, Q14, Q13, Q12, Q1 1. In addition, the base terminals ofthese transistors are respectively connected through the resistors R35through R43 to a negative 8 volt supply source. The emitters of thetransistors Q1 1 through Q19 are connected directly to the negative 8volt supply source. The collectors of transistors Q11 through Q14 arerespectively connected through resistors R44 through R47 to a commonterminal T11 which provides an output terminal AH. Similarly, thecollectors of transistor Q15, Q16, Q17, are respectively connectedthrough resistors R48, R49, R50 to a common terminal T12 which providesan output terminal AG. Finally, the collectors of transistors Q18, Q19are respectively connected through resistors R51, R52 to a commonterminal T13 which provides an output terminal AF.

Reference is now made to FIG. 4 which illustrates in more detail thecircuitry of the time limit adjustment circuit 16 and the time limitcurve generator 18. More particularly, the time limit adjustment circuit16 includes eight input terminals AL, AM, AN, AO, AS, AT, AW, AY whichare each connected to the stationary contacts of a set of fourpotentiometers.

Thus, input terminal A is connected to the commonlyconnected stationarycontact of a set of four potentiometers P1, P2, P3, P4; the inputterminal AN is connected to the stationary contact of a set ofpotentiometers P5, P6, P7, P8; the input terminal AM is connected to thestationary contacts of a set of four potentiometers P9, P10, P11, P12;and the input terminal AL is connected to the commonly-connectedstationary contacts of a set of four potentiometers P13, P14, P15, P16.

Similarly, the input terminal AF is connected to the commonly-connectedstationary contact of a set of four potentiometers P17, P18, P19, P20;the input terminal AT is connected to the commonly-connected stationarycontact of a set of four potentiometers P21, P22, P23, P24; the inputterminal AW is connected to the commonly-connected stationary contact ofa set of four potentiometers P25, P26, P27, P28; and the input terminalAY is connected to the commonly-connected stationary contact of a set offour potentiometers P29, P30, P31, P32. The other stationary contacts ofthe potentiometers P1, P2, P3, P4 are respectively connected to theother stationary contacts of the pontentiometers P17, P18, P19, P20.Similarly, the other stationary contacts of the potentiometers P5, P6,P7, P8 are respectively connected to the other stationary contacts ofthe potentiometers P21, P22, P23, P24. In a like manner, the otherstationary contacts of the potentiometers P9, P10, P11, P12 arerespectively connected to the other stationary contacts of thepotentiometers P25, P26, P27, P28. Finally, the other stationarycontacts of the potentiometers P13, P14, P15, P16 are respectivelyconnected to the other stationary contacts of the po tentiometers P29,P30, P31, P32.

The movable contacts of the potentiometers P1 through P32 arerespectively connected to the anodes of a corresponding member of diodesD61 through D92. Also, the cathodes of the diodes D61 through D76 arerespectively connected to the cathode of the diodes D77 through D92.

As illustrated, the tube limit curve generator 18 in cludes four NPNtransistors Q20, Q21, Q22, Q23 having their collectors connected incommon to an output terminal AB. The base terminal of transistor Q20 isconnected through a resistor R53 to the negative 8 volt supply sourceand is also connected to the cathodes of diodes D61, D65, D69, D73.Similarly, the base of transistor Q21 is connected through a resistorR54 to the negative 8 volt supply source and is also connected to thecathodes of the diodes D62, D66, D70, D74. In a like manner, the base oftransistor Q22 is connected through a resistor R55 to the negative 8volt supply source and is also connected to the cathodes of the diodesD63, D67, D71, D75. Finally, the base of transistor Q3 is connectedthrough a resistor R56 to the negative 8 volt supply source and is alsoconnected to the cathodes of the diodes D64, D68, D72, D76.

The negative 8 volt supply source is coupled directly to all of thejunction points between the series connected potentiometers P1 throughP32. The emitters of. the transistors Q20, Q21, Q22, provide the outputterminals AH, AG, AF, respectively, of the time limit curve generator18.

Reference is now made to FIG. 5 which illustrates in more detail thetube limit decoding circuit 14. This circuit generally comprises fourmode of operation switches, i.e., an overtable switch, OT-l, andovertable low speed switch OTL-l, an undertable low speed switch UTL-2,and a high and low speed switch l-lSS-l. These switches are connectedthrough appropriate relays in order to cause the tube limit curvegenerator 18 to generate a maximum tube input power curve appropriate tothe mode of operation.

More particularly, the switches OT 1, OTL-l are single-pole,single-throw switches with each switch having one terminal connecteddirectly to ground. The other terminal of the switch OT-l is connectedthrough a diode D93 polarized as shown in FIG. 5 to one of the terminalsof a coil 39 of a relay 40. The other terminal of the coil 39 isconnected directly to the positive 28 volt supply source and a diodeD94, polarized as shown in FIG. 5, is coupled across the terminals ofthe relay coil 39.

The other terminal of switch OTL-l is connected through a diode D95,polarized as shown in FIG. 5, to one of the terminals of a coil 42 of arelay 44. The other terminal of coil 42 is connected directly to thepositive 28 volt supply source. A diode D96 polarized as shown in FIG.5, is coupled across the terminals of the relay coil 42.

The other terminal of the switch UTL-2 is connected through a diode D97,polarized as shown in FlG. 5, to the same terminal of relay coil 42which is connected to diode D95.

The relay contacts of relay 40 take the form of doublepole, double-throwcontacts 46, 48. The common contact of the sets of contacts 46, 48 arerespectively connected to a pair of terminals of the high and low speedswitch HSS-l. The switch HSS-l takes the form of a double-pole,double-throw switch. The other contacts of the switch HSS-l areconnected directly to ground.

The relay contacts of relay 44 take the form of four sets ofsingle-pole, double-throw contacts 50, 52, S4, 56. The common terminalof contact set 50 is connected directly to the normally-closed terminalof contact 46 of relay 40, the common terminal of contact set 52 isconnected directly to the normally-closed terminal of contact 48 ofrelay 40, the common terminal of contact set 54 is connected to thenormally-open terminal of contact 46 of relay 40, and the commonterminal of contact set 56 is connected to the normally-open terminal ofcontact 48 of relay 40.

As illustrated, the normally-open terminals of contact sets 50, S2, S4,56 provide the output terminals AL, AM, AN, AO, respectively, of thetube limit decoding circuit 14. Similarly, the normally-closed terminalsof the contact sets 50, 52, S4, 56 provide the output terminals AS, AT,AW, AX of the decoding circuit 14.

Reference is now made to FIG. 6 which is a graphical representation of atypical maximum input tube power curve PC having the time-varying limitcurve which is generated by the tube limit curve generator 18superimposed thereon. More particularly, the maximum input tube powercurve TC represents the maximum voltage times current (KV X MA) as afunction of exposure time which may be applied to the X-ray tube withoutdamaging the X-ray tube. This curve which may be obtained from the tubemanufacturer, will vary according to the mode of operation, i.e., highspeed, low speed, overtable, etc.

The limit curve AC which is generated by the tube limit curve generator18 generally takes the form of a decreasing staircase type signal whichmay be adjusted in amplitude for each time interval to closelyapproximate the value over each time interval of the maximum input tubepower curve PC. In other words, the intervals of time, T1, T2, T3, T4are predetermined fixed intervals of time of equal time duration, andthe voltage, i.e., V1, V2, V3, V4, generated by the tube limit curvegenerator 18 for each interval of time may be adjusted to satisfy theapproximation of the input tube power curve TC.

OPERATION OF THE PROTECTIVE CIRCUIT Prior to the actual operation of theX-ray tube protection circuit, the potentiometers Pl through P32 areadjusted so that the eight sets of curves which are generated by thetube limit curve limit generator 18 closely approximate the maximuminput tube power curves recommended by the tube manufacturer. The eightcurves generally represent different combinations of: types of tubes,size of focal spot, and anode rotation. In other words, there areeight-possible combinations of these parameters which require adifferent maximum input tube power curve. As discussed before, themaximum input tube power curve PC as illustrated in FIG. 6 is a typicalcurve for input power versus exposure time; however, it is to beappreciated that eight different curves each having different amplitudeswith respect to exposure times would be required for the eight possiblemodes of operation.

Thus, in order to generate a signal for overtable, large focal spot,triple-speed operation, the four potentiometers P1, P2, P3, P4, areadjusted to set the voltage amplitude of the curve for the regions RE-l,RE-Z, RE-3, RE-4 respectively. Similarly, in another mode of operation,i.e., in the overtable, large focal spot, singlespeed operation, thepotentiometers P5, -P6, P7, P8 would be adjusted to obtain the desiredsignal amplitudes over the exposure regions RE-l, RE-2, RE-3, RE-4,respectively.

Accordingly, the potentiometers Pl through P32 correspond to thefollowing modes of operation:

Mode of Operation Potentiometers Overtable, large focal spot,

triple-speed P1, P2, P3, P4 Over-table, large focal spot,

single-speed P5, P6, P7, P8 Undertable, large focal spot,

triple-speed P9, P10, P11, P12 Undertable, large focal spot,

single-speed P13, P14, 15, P16 Overtable, small focal spot,

triple-speed P17, P18, P19, P20 Overtable, small focal spot,

single-speed P21, P22, P23, P24

Undertable, small focal spot,

' P25, P26, P27, P28

P29, P30, P31, P32

triple-speed Undertable, small focal spot,

single-speed TABLEI Time Transistor Bits from Time Accumulator 20 RangeTurned on a 16 a: 64 us 156 st: mu

2.77ms Q11 1 l l l l l l 0 2.77-

' 22ms Q12 0 l l l l l l 0 22ms- 44ms Q13 0 0 l l l l l 0 44ms- 88ms Q140 0 0 l l l l 0 88msl76ms Q15 0 O O 0 I t l O l76ms- 352ms Q16 0 0 0 0 0l l 0 352ms- 704ms Q17 0 0 0 0 O 0 I O 0.704ms- 1.4sec. Q18 0 0 O 0 0 00 l l.4sec

2.8sec. Q19

Upon the actuation of each of the transistors Q11 through Q19, adifferent resistance value of a corresponding resistor R44 through R52is coupled into the circuit to thereby cause the generated limit signalAC to decrease in value with elapsed exposure time. Thus, as thetransistors Q1 1 through Q19 are sequentially forward biased, theresistors R44 through R52 are sequentially coupled into the circuit tothereby cause the limit curve AC to take the form of a decreasingstairstep type function with respect to time.

With reference to FIG. 5, the switches 0T-l, UTL-l, UTL-2, 1188-1, andtheir associated relays 40, 44, provide circuitry for switching thedesired set of four potentiometers of the potentiometers P1 through P32into the circuit. In other words, upon closure of the switch OTL-l,assuming the switch I-ISS-l is in the position as indicated, theovertable, large focal spot, triple-speed potentiometers Pl through P4are placed into the circuit to generate a limit curve AC representativeof the maximum input tube power which may be applied to the X-ray tubewhen operated in that mode of operation.

The output current I2 from the tube limit curve generator 18 is appliedacross the resistor R1 to develop a voltage V2 which is proportional tothe maximum allowed input power at the particular time interval of theexposure time. This voltage signal is applied to one of the inputterminals of the comparator circuit 12.

When the X-ray tube supply source is set by the X-ray technician for adesired voltage potential and current signal to be applied to the X-raytube, the resistive arrangements VR-l, VR-2 are simultaneously set todevelop a signal V1 representative of the voltage potential (KV) andcurrent (MA) to be applied to the X-ray tube during an actual exposure.The voltage signal V1 is applied to the other input terminal of thecomparator circuit 12.

Thus, if the voltage V1 remains less than the voltage V2, the signaldeveloped by the comparator circuit remains at a binary thereby causingthe X-ray tube supply source to continue to supply a voltagepotentialsignal to the X-ray tube.

If the signal Vl representative of the value of the signal which isapplied to the X-ray tube exceeds the value of the time-varying signalV2, the signal developed by the comparator circuit 12 changes to abinary 1 signal which is applied through the amplifier 24, inverter 28,NAND gate 30 to thereby interrupt the voltage signal which is applied tothe X-ray tube by the tube supply source.

Accordingly, as long as the value of the programmed signal V1 remainsless than the value of the tube limit signal V2, the exposure willcontinue until it is terminated by either a phototimer or a presettimer. If the value of the programmed signal V1 exceeds the value of thelimit signal V2, the exposure is immediately terminated. Also, themonitoring lamp L-l simultaneously provides a visual indication that theexposure has been terminated.

Although the invention has been shown in connection with a preferredembodiment, it will be readily apparent to those skilled in the art thatvarious changes in form and arrangement of parts may be made to suitrequirements without departing from the spirit and scope of theinvention as defined by the appended claims.

Having thus described my invention, I claim:

1. In an X-ray apparatus a protective circuit for limiting the value ofa signal applied to an X-ray tube and comprising:

signal generating means for generating a limit signal which varies invalue as a function of elapsed exposure time in accordance with thevalue of a maximum tube rating signal;

programming circuit means for developing a program signal having a valuerepresentative of the value of a preselected signal to be applied tosaid X-ray tube;

comparator means for monitoring the limit and program signals fordeveloping an output signal when the value of a said limit signalattains a predetermined value with respect to the value of a saidprogram signal; and,

actuatable circuit means coupled to a said comparator circuit means for,upon receipt of a said output signal, interrupting a signal which isapplied to said X-ray tube.

2. An apparatus as defined in claim 1 wherein said programming meansincludes variable circuit means for altering the value of a said programsignal in accordance with the value of the voltage to be applied to saidX-ray tube.

3. An apparatus as defined in claim 1 wherein said signal generatingmeans includes circuit meansfor generating a said limit signal whichdecreases in value with respect to time in accordance with the value ofa decreasing maximum power rating with respect to exposure time of saidX-ray tube.

4. An apparatus as defined in claim 3 wherein said signal generatingcircuit means includes variable control means for varying the rate atwhich a said limit signal decreases with respect to time.

5. An apparatus as defined in claim 4 wherein said programming meansincludes variable circuit means for altering the value of a said programsignal in accordance with the value of the voltage to be applied to saidX-ray tube.

6. An apparatus as defined in claim 4 wherein said programming meansincludes variable circuit means for altering the value of a said programsignal in accordance with the value of the current to be applied to saidX-ray tube.

7. An apparatus as defined in claim 4 wherein said programming meansincludes first variable circuit means for altering the value of a saidprogram signal in accordance with the value of the voltage to be appliedto said X-ray tube; and,

second variable means for altering the value of a said program signal inaccordance with the value of the current to be applied to said X-raytube.

8. An apparatus as defined in claim 1 wherein said signal generatingmeans includes circuit means for generating a said limit signal whichdecreases in value with respect to time and which varies in the rate ofdecrease in value with respect to time.

9. An apparatus as defined in claim 8 wherein said programming meansincludes first variable circuit means for altering the value of a saidprogram signal in accordance with the value of the voltage to be appliedto said X-ray tube; and,

second variable means for altering the value of a said program signal inaccordance with the value of the current to be applied to said X-raytube.

10. An apparatus as defined in claim 1 wherein said signal generatingmeans includes circuit means for generating a said limit signal whichdecreases in amplitude by a predetermined amount for N intervals of timewhere N is equal or greater than five.

11. An apparatus as defined in claim 1 wherein said signal generatingmeans includes circuit means for generating a said limit signal having aplurality of timed portions each being of a different amplitude.

12. An apparatus as defined in claim ll wherein said programming meansincludes variable circuit means for altering the value of a said programsignal in accordance with the value of the current to be applied to saidX-ray tube.

H3. An apparatus as defined in claim 11 wherein said programming meansincludes first variable circuit means for altering the value of a saidprogram signal in accordance with the value of the voltage to be appliedto said X-ray tube; and,

second variable means for altering the value of a said program signal inaccordance with the value of the current to be applied to said X-raytube.

14. An apparatus as defined in claim 1 wherein said signal generatingmeans includes a plurality of circuit means each for generating a saidlimit signal which varies in value with respect to time in accordancewith one of a plurality of different predetermined tube rating signals;

actuatable switch means for selectively developing a pattern of controlsignals representative of a desired one of said plurality ofpredetermined tube rating signals; and,

second actuatable circuit means coupled to said actuatable switch meansfor, upon receipt of a pattern of control signals, actuating a signalgenerating circuit means corresponding to said received pattern ofcontrol signals to thereby generate a said limit signal which varies inaccordance with a selected one of said plurality of predetermined tuberating signals.

15. An apparatus as defined in claim 14 wherein said programming meansincludes variable circuit means for altering the value of a said programsignal in accordance with the value of the current to be applied to saidX-ray tube.

16. An apparatus as defined in claim 14 wherein said programming meansincludes first variable circuit means for altering the value of a saidprogram signal in accordance with the value of the voltage to be appliedto said X-ray tube; and,

second variable means for altering the value of a said program signal inaccordance with the value of the current to be applied to said X-raytube.

17. An apparatus as defined in claim 1 wherein said signal generatingmeans includes a timer means for developing a plurality of patterns ofsignals each representative of the elapsed time;

a plurality of circuit means each for generating a limit signal whichvaries in accordance with one of a plurality of predetermined timefunctions; and,

second actuatable circuit means coupled to said timer means for, uponreceipt of one of a said plurality of patterns of signals, actuating asignal generating circuit means corresponding to a said received patternof signals to thereby generate a limit signal which varies in accordancewith a selected one of said plurality of time functions.

18. An apparatus as defined in claim 17 wherein said programming meansincludes variable circuit means for altering the value of a said programsignal in accordance with the value of the current to be applied to saidX-ray tube.

19. An apparatus as defined in claim 18 wherein said programming meansincludes first variable circuit means for altering the value of a saidprogram signal in accordance with the value of the voltage to be appliedto said X-ray tube; and,

second variable means for altering the value of a said program signal inaccordance with the value of the current to be applied to said X-raytube.

20. A control system for indicating that the input power which isapplied to an X-ray tube has exceeded a level defined by a maximum tubepower rating which varies in value with respect to time and comprising;

tube limit signal generating means for developing a limit signal whichvaries in value as a function of elapsed exposure time in accordancewith variations in a said maximum power rating of the tube with respectto time; variable programming circuit means for developing a programsignal having a value representative of the value of voltage and currentsignals applied to said X-ray tube;

comparator means for monitoring the limit and program signals fordeveloping an output signal when the value of a said limit signalattains a predetermined value with respect to the value of a saidprogram signal; and

indicator circuit means coupled to a said comparator circuit means for,upon receipt of a said output signal, developing an output indicationthat the tube input power has exceeded the maximum power ratmg.

21. An apparatus as defined in claim 20 wherein said variable programcircuit means includes first variable circuit means for developing afirst signal having a value representative of the value of a voltagepotential to be applied to said X-ray tube;

second variable circuit means for developing a second signal having avalue representative of the value of a current to be applied to saidX-ray tube; and,

resolving circuit means coupled to said first and second circuit meansfor developing a said program signal having a value representative ofthe values of said voltage potential and said current to be applied tosaid X-ray tube.

22. An apparatus as defined in claim 20 wherein said variable programcircuit means includes first variable circuit means for developing afirst signal having a value representative of the value of a current tobe applied to said X-ray tube;

second variable circuit means for developing a second signal having avalue representative of the value of a current to be applied to saidX-ray tube; and,

resolving circuit means coupled to said first and sec ond circuit meansfor developing a said program signal having a value representative ofthe values of said current to be applied to said X-ray tube.

23. An apparatus as defined in claim 20 wherein said tube limit signalgenerating means includes timer means for developing a plurality ofpatterns of control signals each representative of the elapsed exposuretime,

a plurality of signal generating circuits each for developing a limitsignal which decreases in value at v a different predetermined rate;

actuator circuit means'coupled to said timer means for, upon receipt ofsaid plurality of predetermined patterns of signals, actuating acorresponding one of said plurality of signal generating circuits tothereby develop a limit signal which decreases in value at acorresponding predetermined rate.

24. An apparatus as defined in claim 23 wherein said timer meansincludes time accumulator means for developing a plurality of patternsof signals each of which take the form of binary coded decimal signals;and matrix circuit means for converting said binary coded decimalsignals to a plurality of patterns of signals each of which takes theform of analog signals.

25. In an X-ray apparatus a protective circuit for indicating that thevalue of a signal applied to an X-ray tube has reached a time-varyingmaximum tube limit rating and comprising:

an X-ray tube;

a voltage supply source coupled to said X-ray tube for applying anoperating signal to said X-ray tube; means for varying the value of asaid operating signal applied to said X-ray tube;

waveform generating means for developing a limit signal having a valuewhich decreases as a predetermined function with respect to elapsedexposure time for a preselected period of time;

monitor circuit means for developing a signal having a valuerepresentative of a said operating signal applied to said X-ray tube;

comparator means coupled to said waveform generating means and saidcircuit means for developing an output signal when the value of thelimit signal decreases to a predetermined level relative to the monitorsignal; and,

indicator circuit means coupled to said comparator circuit means for,upon receipt of a said output signal, developing an output indicationthat the operating signal has reached a maximum limit value.

26. An apparatus as defined in claim 25 wherein said waveform generatingmeans includes circuit means for generating a said limit signal whichdecreases in value with respect to time and which varies in the rate ofdecrease in value with respect to time.

27. An apparatus as defined in claim 25 wherein said waveform generatingmeans includes a plurality of circuit means each for generating a saidlimit signal which varies in value with respect to time in accordancewith one of a plurality of different predetermined tube rating signals;

actuatable switch means for selectively developing a pattern of controlsignals representative of a desired one of said plurality ofpredetermined tube rating signals; and,

second actuatable circuit means coupled to said actuatable switch meansfor, upon receipt of a pattern of control signals, actuating a signalgenerating circuit means corresponding to said received pattern ofcontrol signals to thereby generate a said limit signal which varies inaccordance with a selected one of said plurality of predetermined tuberating signals.

28. In an X-ray apparatus a protective circuit for monitoring the valueof a signal applied to an X-ray tube from exceeding a time-varyingmaximum tube limit rating and comprising:

signal generating means for generating a limit signal which decreases invalue as a predetermined, nonlinear function with respect to elapsedexposure time in accordance with the value of a time-varying maximumtube rating;

programming circuit means for developing a program signal having a valuerepresentative of the value of a preselected signal applied to a saidX-ray tube; comparator meansfor monitoring the limit and program signalsfor developing an output signal when the value of a said limit signaldecreases to a predetermined value relative to the value of a saidprogram signal; and,

indicator circuit means coupled to said comparator circuit means for,upon receipt of a said output signal, developing. an output indicationthat the operating signal has reached a maximum limit value.

29. An apparatus as defined in claim 28 wherein said signal generatingmeans includes a timer means for developing a plurality of patterns ofsignals each representative of the elapsed time;

a plurality of circuit means each for generating a limit signal whichvaries in accordance with one of a plurality of predetermined timefunctions; and,

second actuatable circuit means coupled to said timer means for, uponreceipt of one of a said plurality of patterns of signals, actuating asignal generating circuit means corresponding to a said received patternof signals to thereby generate a limit signal which varies in accordancewith a selected one of said plurality of time functions.

30. An apparatus as defined in claim 28 wherein said signal generatingmeans includes a plurality of circuit means each for generating a saidlimit signal which varies in value with respect to time in accordancewith one of a plurality of different predetermined tube rating signals;

actuatable switch means for selectively developing a pattern of controlsignals representative of a desired one of said plurality ofpredetermined tube rating signals; and,

second actuatable circuit means coupled to said actuatable switch meansfor, upon receipt of a pattern of control signals, actuating a signalgenerating circuit means corresponding to said received pattern ofcontrol signals to thereby generate a said limit signal which varies inaccordance with a selected one of said plurality of predetermined tuberating signals.

31. A method of protecting an X-ray tube in an X-ray apparatuscomprising the steps of:

generating a limit signal which decreases in value with respect toelapsed exposure time in accordance with a decrease in the value of theamplitude with respect to; exposure time of a maximum signal which maybe applied to the X-ray tube;

developing a program signal having a value representative of the valueof a preselected signal to be applied to said X-ray tube;

comparing the values of the limit signal and program signal;

developing an output signal if the value of said program signal exceedsthe value of said limit signal; and,

interrupting a signal applied to said X-ray tube in response to thereceipt of an output signal.

1. In an X-ray apparatus a protective circuit for limiting the value ofa signal applied to an X-ray tube and comprising: signal generatingmeans for generating a limit signal which varies in value as a functionof elapsed exposure time in accordance with the value of a maximum tuberating signal; programming circuit means for developing a program signalhaving a value representative of the value of a preselected signal to beapplied to said X-ray tube; comparator means for monitoring the limitand program signals for developing an output signal when the value of asaid limit signal attains a predetermined value with respect to thevalue of a said program signal; and, actuatable circuit means coupled toa said comparator circuit means for, upon receipt of a said outputsignal, interrupting a signal which is applied to said X-ray tube.
 2. Anapparatus as defined in claim 1 wherein said programming means includesvariable circuit means for altering the value of a said program signalin accordance with the value of the voltage to be applied to said X-raytube.
 3. An apparatus as defined in claim 1 wherein said signalgenerating means includes circuit means for generating a said limitsignal which decreases in value with respect to time in accordance withthe value of a decreasing maximum power rating with respect to exposuretime of said X-ray tube.
 4. An apparatus as defined in claim 3 whereinsaid signal generating circuit means includes variable control means forvarying the rate at which a said limit signal decreases with respect totime.
 5. An apparatus as defined in claim 4 wherein said programmingmeans includes variable circuit means for altering the value of a saidprogram signal in accordance with the value of the voltage to be appliedto said X-ray tube.
 6. An apparatus as defined in claim 4 wherein saidprogramming means includes variable circuit means for altering the valueof a said program signal in accordance with the value of the current tobe applied to said X-ray tube.
 7. An apparatus as defined in claim 4wherein said programming means includes first variable circuit means foraltering the value of a said program signal in accordance with the valueof the voltage to be applied to said X-ray tube; and, second variablemeans for altering the value of a said program signal in accordance withthe value of the current to be applied to said X-ray tube.
 8. Anapparatus as defined in claim 1 wherein said signal generating meansincludes circuit means for generating a said limit signal whichdecreases in value with respEct to time and which varies in the rate ofdecrease in value with respect to time.
 9. An apparatus as defined inclaim 8 wherein said programming means includes first variable circuitmeans for altering the value of a said program signal in accordance withthe value of the voltage to be applied to said X-ray tube; and, secondvariable means for altering the value of a said program signal inaccordance with the value of the current to be applied to said X-raytube.
 10. An apparatus as defined in claim 1 wherein said signalgenerating means includes circuit means for generating a said limitsignal which decreases in amplitude by a predetermined amount for Nintervals of time where N is equal or greater than five.
 11. Anapparatus as defined in claim 1 wherein said signal generating meansincludes circuit means for generating a said limit signal having aplurality of timed portions each being of a different amplitude.
 12. Anapparatus as defined in claim 11 wherein said programming means includesvariable circuit means for altering the value of a said program signalin accordance with the value of the current to be applied to said X-raytube.
 13. An apparatus as defined in claim 11 wherein said programmingmeans includes first variable circuit means for altering the value of asaid program signal in accordance with the value of the voltage to beapplied to said X-ray tube; and, second variable means for altering thevalue of a said program signal in accordance with the value of thecurrent to be applied to said X-ray tube.
 14. An apparatus as defined inclaim 1 wherein said signal generating means includes a plurality ofcircuit means each for generating a said limit signal which varies invalue with respect to time in accordance with one of a plurality ofdifferent predetermined tube rating signals; actuatable switch means forselectively developing a pattern of control signals representative of adesired one of said plurality of predetermined tube rating signals; and,second actuatable circuit means coupled to said actuatable switch meansfor, upon receipt of a pattern of control signals, actuating a signalgenerating circuit means corresponding to said received pattern ofcontrol signals to thereby generate a said limit signal which varies inaccordance with a selected one of said plurality of predetermined tuberating signals.
 15. An apparatus as defined in claim 14 wherein saidprogramming means includes variable circuit means for altering the valueof a said program signal in accordance with the value of the current tobe applied to said X-ray tube.
 16. An apparatus as defined in claim 14wherein said programming means includes first variable circuit means foraltering the value of a said program signal in accordance with the valueof the voltage to be applied to said X-ray tube; and, second variablemeans for altering the value of a said program signal in accordance withthe value of the current to be applied to said X-ray tube.
 17. Anapparatus as defined in claim 1 wherein said signal generating meansincludes a timer means for developing a plurality of patterns of signalseach representative of the elapsed time; a plurality of circuit meanseach for generating a limit signal which varies in accordance with oneof a plurality of predetermined time functions; and, second actuatablecircuit means coupled to said timer means for, upon receipt of one of asaid plurality of patterns of signals, actuating a signal generatingcircuit means corresponding to a said received pattern of signals tothereby generate a limit signal which varies in accordance with aselected one of said plurality of time functions.
 18. An apparatus asdefined in claim 17 wherein said programming means includes variablecircuit means for altering the value of a said program signal inaccordance with the value of the current to be applied to said X-raytube.
 19. An apparatus as defined in claim 18 wherein said prOgrammingmeans includes first variable circuit means for altering the value of asaid program signal in accordance with the value of the voltage to beapplied to said X-ray tube; and, second variable means for altering thevalue of a said program signal in accordance with the value of thecurrent to be applied to said X-ray tube.
 20. A control system forindicating that the input power which is applied to an X-ray tube hasexceeded a level defined by a maximum tube power rating which varies invalue with respect to time and comprising; tube limit signal generatingmeans for developing a limit signal which varies in value as a functionof elapsed exposure time in accordance with variations in a said maximumpower rating of the tube with respect to time; variable programmingcircuit means for developing a program signal having a valuerepresentative of the value of voltage and current signals applied tosaid X-ray tube; comparator means for monitoring the limit and programsignals for developing an output signal when the value of a said limitsignal attains a predetermined value with respect to the value of a saidprogram signal; and indicator circuit means coupled to a said comparatorcircuit means for, upon receipt of a said output signal, developing anoutput indication that the tube input power has exceeded the maximumpower rating.
 21. An apparatus as defined in claim 20 wherein saidvariable program circuit means includes first variable circuit means fordeveloping a first signal having a value representative of the value ofa voltage potential to be applied to said X-ray tube; second variablecircuit means for developing a second signal having a valuerepresentative of the value of a current to be applied to said X-raytube; and, resolving circuit means coupled to said first and secondcircuit means for developing a said program signal having a valuerepresentative of the values of said voltage potential and said currentto be applied to said X-ray tube.
 22. An apparatus as defined in claim20 wherein said variable program circuit means includes first variablecircuit means for developing a first signal having a valuerepresentative of the value of a current to be applied to said X-raytube; second variable circuit means for developing a second signalhaving a value representative of the value of a current to be applied tosaid X-ray tube; and, resolving circuit means coupled to said first andsecond circuit means for developing a said program signal having a valuerepresentative of the values of said current to be applied to said X-raytube.
 23. An apparatus as defined in claim 20 wherein said tube limitsignal generating means includes timer means for developing a pluralityof patterns of control signals each representative of the elapsedexposure time; a plurality of signal generating circuits each fordeveloping a limit signal which decreases in value at a differentpredetermined rate; actuator circuit means coupled to said timer meansfor, upon receipt of said plurality of predetermined patterns ofsignals, actuating a corresponding one of said plurality of signalgenerating circuits to thereby develop a limit signal which decreases invalue at a corresponding predetermined rate.
 24. An apparatus as definedin claim 23 wherein said timer means includes time accumulator means fordeveloping a plurality of patterns of signals each of which take theform of binary coded decimal signals; and matrix circuit means forconverting said binary coded decimal signals to a plurality of patternsof signals each of which takes the form of analog signals.
 25. In anX-ray apparatus a protective circuit for indicating that the value of asignal applied to an X-ray tube has reached a time-varying maximum tubelimit rating and comprising: an X-ray tube; a voltage supply sourcecoupled to said X-ray tube for applying an operating signal to saidX-ray tube; meanS for varying the value of a said operating signalapplied to said X-ray tube; waveform generating means for developing alimit signal having a value which decreases as a predetermined functionwith respect to elapsed exposure time for a preselected period of time;monitor circuit means for developing a signal having a valuerepresentative of a said operating signal applied to said X-ray tube;comparator means coupled to said waveform generating means and saidcircuit means for developing an output signal when the value of thelimit signal decreases to a predetermined level relative to the monitorsignal; and, indicator circuit means coupled to said comparator circuitmeans for, upon receipt of a said output signal, developing an outputindication that the operating signal has reached a maximum limit value.26. An apparatus as defined in claim 25 wherein said waveform generatingmeans includes circuit means for generating a said limit signal whichdecreases in value with respect to time and which varies in the rate ofdecrease in value with respect to time.
 27. An apparatus as defined inclaim 25 wherein said waveform generating means includes a plurality ofcircuit means each for generating a said limit signal which varies invalue with respect to time in accordance with one of a plurality ofdifferent predetermined tube rating signals; actuatable switch means forselectively developing a pattern of control signals representative of adesired one of said plurality of predetermined tube rating signals; and,second actuatable circuit means coupled to said actuatable switch meansfor, upon receipt of a pattern of control signals, actuating a signalgenerating circuit means corresponding to said received pattern ofcontrol signals to thereby generate a said limit signal which varies inaccordance with a selected one of said plurality of predetermined tuberating signals.
 28. In an X-ray apparatus a protective circuit formonitoring the value of a signal applied to an X-ray tube from exceedinga time-varying maximum tube limit rating and comprising: signalgenerating means for generating a limit signal which decreases in valueas a predetermined, non-linear function with respect to elapsed exposuretime in accordance with the value of a time-varying maximum tube rating;programming circuit means for developing a program signal having a valuerepresentative of the value of a preselected signal applied to a saidX-ray tube; comparator means for monitoring the limit and programsignals for developing an output signal when the value of a said limitsignal decreases to a predetermined value relative to the value of asaid program signal; and, indicator circuit means coupled to saidcomparator circuit means for, upon receipt of a said output signal,developing an output indication that the operating signal has reached amaximum limit value.
 29. An apparatus as defined in claim 28 whereinsaid signal generating means includes a timer means for developing aplurality of patterns of signals each representative of the elapsedtime; a plurality of circuit means each for generating a limit signalwhich varies in accordance with one of a plurality of predetermined timefunctions; and, second actuatable circuit means coupled to said timermeans for, upon receipt of one of a said plurality of patterns ofsignals, actuating a signal generating circuit means corresponding to asaid received pattern of signals to thereby generate a limit signalwhich varies in accordance with a selected one of said plurality of timefunctions.
 30. An apparatus as defined in claim 28 wherein said signalgenerating means includes a plurality of circuit means each forgenerating a said limit signal which varies in value with respect totime in accordance with one of a plurality of different predeterminedtube rating signals; actuatable switch means for selectively developinga pattern of control signals representative of a desiRed one of saidplurality of predetermined tube rating signals; and, second actuatablecircuit means coupled to said actuatable switch means for, upon receiptof a pattern of control signals, actuating a signal generating circuitmeans corresponding to said received pattern of control signals tothereby generate a said limit signal which varies in accordance with aselected one of said plurality of predetermined tube rating signals. 31.A method of protecting an X-ray tube in an X-ray apparatus comprisingthe steps of: generating a limit signal which decreases in value withrespect to elapsed exposure time in accordance with a decrease in thevalue of the amplitude with respect to exposure time of a maximum signalwhich may be applied to the X-ray tube; developing a program signalhaving a value representative of the value of a preselected signal to beapplied to said X-ray tube; comparing the values of the limit signal andprogram signal; developing an output signal if the value of said programsignal exceeds the value of said limit signal; and, interrupting asignal applied to said X-ray tube in response to the receipt of anoutput signal.